July 10, 2013

New Research Challenges 50 Years Of Muscle Knowledge

For the past 50 years, scientists and health professionals have operated with the knowledge that biceps are packed with long ropes of filaments. When the bicep is used to lift a heavy object or flex, these filaments were understood to shorten or contract the muscle and give it its power.

A new study from the University of Washington (UW) challenges this notion, saying that while the general idea of filaments contracting is the same, the shape of these filaments is quite different. Instead of long strands of contracting and shortening filaments straight up and down the length of the muscle, a mesh arrangement of filaments spreads out across the muscle.

According to C. David Williams, a postdoctoral researcher at Harvard who earned his doctorate while at UW, this action is responsible for the muscle force measured by scientists for years. The new study is published in the Royal Society journal Proceedings of the Royal Society B.

"The predominant thinking of the last 50 years is that 100 percent of the muscle force comes from changes as muscles shorten and myosin and actin filaments overlap. But when we isolated the effects of filament overlap we only got about half the change in force that physiologists know muscles are capable of producing," said Williams.

The mesh of filaments fans out into a lattice that is also responsible for the lifting power of biceps and the force generated by other muscles. Williams' discovery doesn't only cover the shape of the filaments in the muscles.

According to Thomas Daniel, a professor of biology at UW and co-author of the resulting paper, this discovery could have long lasting ramifications on the understanding of muscles in the body.

"One of the major discoveries that David Williams brought to light is that force is generated in multiple directions, not just along the long axis of muscle as everyone thinks, but also in the radial direction. This aspect of muscle force generation has flown under the radar for decades and is now becoming a critical feature of our understanding of normal and pathological aspects of muscle," said Daniel.

To better understand his new hypothesis, Williams created a 3D computer model to see the geometry and physics at work inside a muscle wrapped in mesh filament. According to Williams, his research wouldn't have been possible without the ability to render these computer models and watch the way these filaments work together. With these computer models in place, the research team then validated their hypothesis with the flight muscle of a moth. This muscle has been compared to the human cardiac muscle in the past. The same lattice spacing effect occurred in the flight muscle, leading the team to believe that the same effect can be seen in other muscles of the body as well.

"In the heart especially, because the muscle surrounds the chambers that fill with blood, being able to account for forces that are generated in several directions during muscle contraction allows for much more accurate and realistic study of how pressure is generated to eject blood from the heart," said co-author Michael Regnier.

"The radial and long axis forces that are generated may be differentially compromised in cardiac diseases and these new, detailed models allow this to be studied at a molecular level for the first time."